The present invention relates to a method for the preparation of a high-quality powder of amorphous silicic acid or, more particularly, to an efficient method for the preparation of a high-quality amorphous silicic acid in the form of a powder having usefulness as an adjuvant ingredient in various kinds of cosmetic and toiletry preparations, as a carrier material of a catalyst used in a chemical reaction to support the catalytically active ingredient thereon and as a filtration aid capable of having a high filtration coefficient in the process of solid-liquid separation by virtue of the high acid resistance, high whiteness and large specific surface area.
It is an established chemical engineering technology in the prior art that, in conducting solid-liquid separation by filtration of a liquid suspension containing suspended fine particles which may readily cause clogging of the filtering material, e.g., filter paper and filter cloth, or a suspension of which the suspended particles are extremely fine and the concentration thereof in the suspension is very low, a filtration aid in the form of a fine powder, which typically consists of a diatomaceous earth, is used by adding to the suspension to be filtered or by forming a coating layer on the filtering material with an object to increase the rate of filtration or to increase the clarity of the filtrate obtained by filtration.
Since diatomaceous earth is a naturally occurring mineral product, the diatomaceous earth-based filtration aids are greatly diversified in the types and grades relative to the particle diameter, crystallite morphology and other characteristics to exhibit a large difference in the filtration behavior. Accordingly, it is usual that a very strict and precise quality control is essential in the manufacturing process of a diatomaceous earth-based filtration aid including the steps of pulverization, particle size classification and the like unavoidably leading to an increase in the manufacturing costs.
Moreover, some countries in the world have a problem relative to the resource abundance and quality of the ore beds of diatomaceous earth not to ensure stable supply of high-quality diatomaceous earth products as a base material of filtration aids. In addition, as an inherence of a naturally deposited mineral product, diatomaceous earth always contains a considerable amount of impurities including organic matters and ferrous matters so that the applicability of a diatomaceous earth-based filtration aid is more or less limited in the field of the pharmaceutical industry. Accordingly, it is almost always necessary that a diatomaceous earth-based filtration aid is subjected to an elaborate pretreatment in order to minimize contamination of the material filtered therewith by an organic matter originating in the filtration aid. The ferrous impurities are particularly undesirable in the filtration aids used in the food industry and petroleum industry in view of the unavoidable coloring of the products by the ferrous impurities.
On the other hand, extensive investigations are now under way for the use of calcium silicate particles obtained by a hydrothermal reaction of a siliceous material and a calcific material as a filtration aid. Although calcium silicate products have a well controlled particle diameter and crystalline morphology suitable for a particular application by adequately selecting the preparation conditions even without undertaking expensive procedures of pulverization and particle size classification and also can be free from organic and ferrous impurities by using appropriate starting materials, a most serious problem in the calcium silicate-based filtration aids is their low acid resistance which limits their use under an acidic condition.
In view of the above described problems in the calcium silicate-based filtration aids in the prior art, the inventor has disclosed, in Japanese Patent Kokai 7-206423 and 8-245215, an improved method for the preparation thereof in which a siliceous material and a calcific material are blended in a specified proportion and the blend is subjected to a hydrothermal reaction to form calcium silicate which is subjected to a heat treatment at a temperature in the range from 800 to 1200.degree. C., and, in Japanese Patent Kokai 8-245215 and 9-255323, a method in which a calcium silicate powder obtained by a hydrothermal reaction is subjected to a carbonation treatment followed by a treatment in an acidic aqueous solution. The inventor has further developed a method for the preparation of a calcium silicate-based filtration aid in which an aqueous slurry of calcium silicate particles obtained by a hydrothermal reaction under specified conditions is admixed with an acid to effect decalcification of the calcium silicate particles.
Further, Japanese Patent Kokai 51-125699 discloses a method in which calcium silicate particles are subjected to a carbonation reaction to effect a decomposition reaction thereof into amorphous silicic acid and calcium carbonate followed by an acid treatment to dissolve away the calcium carbonate leaving amorphous silicic acid as a solid product.
A serious problem in the above described method is that, when a wet particles of the amorphous silicic acid obtained by the decalcification treatment are dried, the wet cake of the particles greatly shrinks and is consolidated so that an amorphous silicic acid powder retaining a highly porous structure, suitable for use as a filtration aid, can no longer be obtained. The mechanism for the drying shrinkage of the amorphous silicic acid cake is presumably that, when the calcium silicate obtained by the hydrothermal reaction is treated with an acid to dissolve away the calcific constituent to form amorphous silicic acid, highly hydrophilic silanolic hydroxyl groups are formed which combine a large amount of water onto the silicic acid particles so that, when the water is removed by evaporation in the drying step, a very large coherent force of several hundreds of kg/cm.sup.2 due to the large surface tension of water acts between the secondary particles or between the primary particles forming the secondary particles of the amorphous silicic acid resulting in shrinkage and consolidation of the wet cake of amorphous silicic acid particles.
The above mentioned problem of shrinkage of a wet cake of the amorphous silicic acid particles by drying can be partly solved by repeatedly washing the wet particles with acetone before drying. The shrinkage-preventing effect of this method, however, is far from complete and, when the starting calcium silicate has particularly low crystallinity, the amorphous silicic acid obtained therefrom has a great specific surface area so that the effectiveness of the method is very limited if not to mention the economical problem due to the costs for the use of a large volume of expensive acetone.
As an alternative method for drying a wet cake of amorphous silicic acid particles, an aqueous slurry of the particles is subjected to spray drying. This method, however, is economically very disadvantageous because the spray nozzle of the spray drier is readily clogged by the particles unless the solid content of the aqueous slurry is unduly low to be, for example, 2 to 3% by weight necessitating consumption of a very large quantity of thermal energy for evaporation of water.